US6364899B1 - Heat pipe nerve cooler - Google Patents

Abstract

The invention provides a method and apparatus for producing reversible focal hypothermia of the nervous system to control chronic pain. Nerve conduction is blocked by mild cooling (0 to 25° C.), or hypothermia. At these temperatures, nerve tissue is not destroyed and recovers completely when cooling is terminated, such that the treatment is reversible. By blocking conduction in pain nerves, pain sensation is eliminated in a manner analogous to drugs such as lidocaine that also block nerve conduction to provide anesthesia. The invention can be applied to a variety of conditions such as urge incontinence, muscle spasticity, and epilepsy. Many of these disorders are mediated by nerve and nervous tissue that could be interrupted by cooling. In addition, neurologic dysfunction found in multiple sclerosis may improve by cooling of the nerves. The method and apparatus may be used to cool areas of the nervous system affected by multiple sclerosis to allow more normal functions.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of the following U.S. Pat. applications: U.S. patent application Ser. No. 09/012,287, filed Jan. 23, 1998, now U.S. Pat. No. 6,051,019 and entitled “Selective Organ Hypothermia Method and Apparatus”; U.S. patent application Ser. No. 09/047,012, filed Mar. 24, 1998, now U.S. Pat. No. 5,957,963 and entitled “Improved Selective Organ Hypothermia Method and Apparatus”; U.S. patent application Ser. No. 09/052,545, filed Mar. 31, 1998, and entitled “Circulating Fluid Hypothermia Method and Apparatus”; U.S. patent application Ser. No. 09/103,342, filed Jun. 23, 1998, now U.S. Pat. No. 6,096,068 and entitled “A Selective Organ Cooling Catheter and Method of Using the Same”; U.S. patent application Ser. No. 09/215,038, filed Dec. 16, 1998, and entitled “An Inflatable Catheter for Selective Organ Heating and Cooling Catheter and Method of Using the Same”; U.S. patent application Ser. No. 09/215,040, filed Dec. 16, 1998, and entitled “Method and Device for Applications of Selective Organ Cooling”; and U.S. patent application Ser. No. 09/262,805, filed Mar. 4, 1999, and entitled “A Selective Organ Cooling Catheter with Guide Wire Apparatus”.

REFERENCE TO FEDERAL FUNDING

Not Applicable.

BACKGROUND OF THE INVENTION

Pain sensation is mediated by nerve fibers. Nerve fibers extend to the brain via the spinal cord which forms a portion of the central nervous system. Referring to FIG. 1, thespinal cord12 extends from the brain to the level of the second lumbar vertebra, at which point the spinal cord branches to numerous individual roots. Throughout the length of the spinal cord, the same is encased in the vertebral canal.Nerves14 branch off at regular intervals.

A number of types of nerves are disposed within the posteriorgray horn16. Two types of pain sensing nerves have been identified: A_δ and C. Referring to FIGS. 2 and 3, A_δfibers18 are disposed within regions I and V and the same produce a rapid initial and intense response to painful stimuli.C fibers20 are disposed within region II and produce a more blunted but prolonged response.C fibers20 are believed to be responsible for many chronic pain syndromes.

A_δfibers18 andC fibers20 are connected to thespinal cord20 via the dorsal root22 (referring back to FIG.1). Thedorsal root22 is a bundle of nerves that enters the dorsal aspect of thespinal cord12. The sensory nerves from one particular body region, such as the right leg, may be split among several dorsal root nerve bundles spaced along the length of thespinal cord12.

Pain is conducted via fibers of the peripheral nervous system to the central nervous system, or nerves in the spinal cord. The pain signal is conducted up nerve tracts of the spinal cord to the pain sensing areas of the brain (i.e., the thalamus). The transmission of the pain signal from the peripheral nerves to the central nerves takes place in the synapses of the posteriorgray horn region16 of thespinal cord12. A synapse is a neuron-to-neuron transmission of a signal by a chemical mediator that traverses a small gap between two axon terminals.

As noted above, many A_δfibers18 andC fibers20 synapse in the most superficial, or dorsal, region of the dorsal gray horn known as zones1 and2. The synaptic region of theC fibers20 is also known as the substantia gelatinosa. Various treatments directed at these fibers and these anatomical locations, can be and are used to treat pain syndromes.

An estimated 15 million Americans suffer from chronic intractable pain. 50% of persons with terminal illness have significant pain and 10% require a surgical procedure to treat the pain. $80 billion is spent annually in the United States-on chronic pain.

Current therapy for chronic pain can be divided into two categories: medical and surgical. Medical therapy is the administration of drugs ranging from Tylenol® to morphine. Morphine and its analogs are used in cases of severe pain and terminal illness. These drugs have many serious side effects such as sedation, confusion, constipation, and depression of respiration. The more severe the pain, the higher the dosage of the drug and the more significant the side effects. In addition, tolerance to these compounds develops, and escalating doses are required to achieve pain control.

Surgical therapy can range from the implantation of drug infusion devices to the ablation, or destruction, of nerves. Ablation of nervous tissue is irreversible and can cause permanent loss of function of organs and limbs. One type of surgical treatment is known as Dorsal Root Entry Zone (“DREZ”) ablation. The DREZ is shown in FIG. 1 as DREZ24. While DREZ ablation is effective at treating pain, it can also result in significant limb and organ dysfunction. Drug infusion into the spinal cord using implanted devices can reduce drug side effects, however they do not eliminate side effects entirely nor solve the problem of tolerance. These approaches require significant surgical procedures; often, terminally ill patients are not good candidates for surgery.

Nerve stimulators are also used for pain control. These electrical devices work indirectly by stimulating nerve fibers that inhibit conduction pain fibers. It is known to place devices such as nerve stimulators surgically or percutaneously and they may be placed directly adjacent to the spinal cord. For example, U.S. Pat. No. 5,643,330 to Holsheimer et al., issued Jul. 1, 1997, and entitled “Multichannel Apparatus for Epidural Spinal Cord Stimulation”, discloses placing an epidural spinal cord stimulator adjacent to spinal cord dura mater.

Stimulators are relatively ineffective in controlling pain. This may be in part due to the indirect mechanism of action. Further, they can cause dysthesias and paresthesias (neurologenic pain) due to the stimulation of nerve fibers.

There is a need for a method and apparatus to combat pain, especially chronic pain, which do not suffer from the drawbacks of current medical and surgical therapies.

SUMMARY OF THE INVENTION

In one aspect, the invention is directed to a method of cooling a portion of a spinal cord of a patient. The method includes delivering a portion of a heat pipe to a spinal cord of a patient, the heat pipe including an evaporator and a condenser, including disposing the evaporator at least in partial thermal communication with the spinal cord. The evaporator is cooled by passing a working fluid between the evaporator and the condenser.

Implementations of the invention may include one or more of the following. The delivering may further include disposing the evaporator at least in partial thermal communication with the dorsal root entry zone of the spinal cord. The working fluid may be passed between the evaporator and the condenser through a conduit, and the conduit may be a tube or wick structure, for example. The condenser may be implanted within a patient or may be located externally of a patient. The condenser may have an insulated lower chamber into which the conduit enters and an upper chamber into which the return tube enters, the lower and upper chambers separated by a porous structure, and may further include passing the working fluid in gaseous form from the evaporator through the return tube within the conduit to the upper chamber, condensing the working fluid at least partially from the gaseous form into the liquid form, passing the working fluid from the upper chamber to the lower chamber through the porous structure, and passing the condensed working fluid from the lower chamber to the evaporator through the conduit. Another implementation may include disposing the upper chamber in thermal communication with a cold source. The evaporator may be disposed adjacent the dura mater, or between the spinal cord and the dura mater, or on the side of the dura mater opposite the spinal cord.

In another aspect, the invention is directed to an apparatus for cooling a portion of tissue. The apparatus includes an evaporator to be placed in thermal communication with a portion of tissue; a condenser disposed in thermal communication with a source or sink of heat, the condenser including an upper chamber and a lower chamber; and a conduit disposed between the evaporator and the condenser, the conduit including a wick structure, to communicate working fluid between the two. An implementation of the invention may include providing a porous structure to separate the lower chamber from the upper chamber.

Advantages of the invention include one or more of the following. The invention provides for control of chronic pain in an effective manner. The processes used to achieve hypothermia to control pain are reversible. Nerve tissue is not destroyed as in certain other techniques. Nerve tissue recovers completely when the processes are stopped. The invention allows for treatment of not only chronic pain but also urge incontinence, muscle spasticity, epilepsy, and may even be of benefit in treating multiple sclerosis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of the spinal cord.

FIG. 2 is a schematic cross-section of the spinal cord showing the anterior and posterior gray horn.

FIG. 3 is more detailed schematic cross-section of a portion of the spinal cord, showing the posterior horn and layers of nerve fibers therein.

FIG. 4 is a schematic cross-sectional side view of an embodiment of the invention, which may be implanted into a patient suffering chronic pain.

FIG. 5 is a schematic cross-sectional side view of an alternative embodiment of the invention, which may be implanted into a patient suffering chronic pain.

FIG. 6 is a schematic cross-sectional side view of another alternative embodiment of the invention, which may be implanted into a patient suffering chronic pain.

FIG. 7 is a schematic view of the embodiments of the invention shown in FIGS. 4-6 including a schematic of the same's placement within a patient.

FIG. 8 is a schematic view of an alternative embodiment of the invention including a schematic of the same's placement within a patient.

FIG. 9 is a more detailed schematic view of the placement of the invention alongside a patient's spinal cord.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides, in one embodiment, a cooling catheter or cooling patch that can be placed on nerve fibers or tissue. When nerve tissue is cooled (+2° to +20° C.), conduction therethrough is stopped. Synaptic transmission is susceptible to termination by cooling, with near complete blockage of pain transmission occurring at +20° C. A δfibers are more susceptible to reduction of conduction via cooling and will be affected by warmer temperatures than C-fibers. For example, some A_δ fibers will cease to conduct at +8° C. whereas the conduction of some C-fibers is substantially blocked at +3° C. This conduction block is known to be reversible. Normal conduction returns once the nerve warms.

In one method of controlling pain, described in more detail below, the cooling patch or catheter is placed parallel along the dorsalroot entry zone24 in contact with thespinal cord12 or on thedura26, a membrane that surrounds the cord (see FIGS.1 and9). The cooling section of the catheter or patch could be 5 to 10 cm long, or greater, and would stretch along several or many DREZs. This would substantially ensure the treatment of all pain fibers for a given body area that is the source of pain.

By placing the cooling device in thespinal cord12, the synapses at the DREZ can be affected. Since these synapses are susceptible to termination or reduction of conduction at relatively warm temperatures, the temperature of the cooling device can be maintained at a reasonably warm temperature. For example, the surface of the cooling catheter may be maintained at +5° to +10° C. to produce cooling to +20° C. at the depth of the substantia gelatinosa28 (FIG.2), or 2-3 mm, at theDREZ24.

One method of cooling employs a passive two-phase heat transfer device, or heat pipe. Referring to FIG. 4, aheat pipe101 includes three basic parts: anevaporator106, anintervening connecting conduit104, and acondenser102. Theevaporator106 and thecondenser102 are connected to each other by theconduit104.

Theconduit104, which is insulated, allows a coolant to flow between theevaporator106 and thecondenser102. Theconduit104 may employ a variety of structures. In FIG. 4, acapillary tube122 is shown in which coolant flows by capillary forces. In FIG. 5, a conventional wick structure is shown. In FIG. 6, a cylindrical wick structure with a central return lumen is shown. In general, as shown in FIGS. 4-6, theconduit104 includes atube114.Tube114 defines a return path for gaseous coolant as will be described below.

In theheat pipe101, heat enters from the body tissue and is absorbed by theevaporator106. A liquid coolant such as a freon, within theevaporator106, boils and absorbs the heat input, resulting in cooling. The vaporized freon then returns to thecondenser102 via areturn tube116 defined bytube114 within theconduit104. At thecondenser102, heat is removed, either by ambient air heat exchange or by cooling from another source. The cooled coolant condenses the gaseous coolant and the same then flows back down theconduit104 to theevaporator106.

Thecondenser102 may be a small hollow metallic disc made from titanium, stainless steel, or other similar metals. The disk acts as a condenser and reservoir for a freon or other such working fluid. The disc has two chambers, anupper chamber108 and alower chamber118. Thelower chamber118 may be insulated by an evacuatedspace128 or other such insulation. There is no insulation on theupper chamber108 of the disk orcondenser102. A porous/sintered disk110 may optionally be used to separate the two halves. Theconduit104 enters the insulatedlower chamber118 or porous structure ordisc110. Theevaporator conduit116 enters the upper chamber108 (i.e., the uninsulated half of the disk). The connection of theevaporator conduit116 into theupper chamber108 is indicated in FIGS. 4-6, although some details of the connection are omitted for clarity. At least oneheat transfer fin152 may be provided within the upper chamber to assist in the conduction of heat away fromporous structure110 to the cold source described below (for clarity, thisfin152 is only shown in FIG.4).

In FIG. 4, theconduit104 includes acapillary tube122. Thecapillary tube122 causes capillary forces to move the liquid coolant from thecondenser102 to theevaporator106. The liquid inlet to thecapillary tube122 may be entirely within alower chamber118, described in more detail below, entirely within aporous disc110, described in more detail above, or partly in both. In FIG. 4, the last embodiment is shown. In other words, liquid coolant may entertube122 through either of theporous disc110 or thelower chamber118.

In FIG. 5, theconduit104 includes awick structure122′. The wick structure “wicks” the liquid coolant to theevaporator106. Of course, it is understood that wick structures also employ capillary action, but in this embodiment the wick structure is distinguished from a capillary tube per se. Like the embodiment of FIG. 4, thewick structure122′ may be connected either to thelower chamber118,porous disc110, or both. Also in this embodiment, thelower chamber118 should be sealed so thatonly wick structure122′ (and of course porous disc110) may be inlets and outlets. In other words, evaporated gaseous coolant should be prohibited from enteringlower chamber118. The same is true ofcapillary tube122.

In FIG. 6, acylindrical wick structure122″ is shown that provides an additional embodiment of the invention. In this embodiment, thewick structure122″ approximately matches the inner diameter of theconduit104. In this way, thewick structure122″ is provided with more surface area and volume with which to wick coolant. The same travels down thewick structure122″ to the evaporator. Once evaporated, the gaseous coolant may travel in thecentral lumen116 defined by thewick structure122″ itself back to thecondenser102. Of course, thewick structure122″ in this embodiment is shaped such that coolant may reach even the upper portions of thewick structure122″ (adjacent upper chamber108) without entering theupper chamber108. Nevertheless, most of the coolant may still travel along the portion of the wick structure adjacent thelower chamber118. As above, the wick structure may contact the lower chamber118 (as shown in FIG. 6) or may alternatively contact theporous disc110, or both.

Theevaporator106 may be, e.g., a 1-2 mm outer diameter catheter disposed along the spinal cord, and may be, e.g., 10 to 15 cm in length. Theevaporator106 may havemetal foil windows126 that respectively align with the plurality of DREZ24 thereby enhancing heat transfer. Theevaporator106 catheter can be made from polyimide and themetal foil windows126 may be made of platinum or platinum iridium. It should be clear to one of skill in the art that the relative dimensions of theevaporator106 in FIGS. 4-6 are greatly exaggerated and that most feasible such evaporators would have a ratio of length to width that is much greater than that shown in the figures.

Theevaporator106 is connected to thecondenser102 by theconduit104. The conduit may reside in the tissue between the skin and the spinal cord. An end of theconduit104 distal ofevaporator106 may be located between the skin and the spinal cord, and more preferably near the skin so as to allow thermal energy to be passed from the skin to the conduit and condenser, as well as vice-versa. In a separate embodiment, described in more detail below,conduit104 may extend through a percutaneous incision to a region external of the body. It is also noted that theevaporator106 may include a portion of theconduit104 for better delivery of the coolant to the heat transfer portions of the evaporator.

In use, theevaporator106 is inserted along and adjacent to thespinal cord12 percutaneously with a needle introducer. The needle introducer allows theevaporator106 to be disposed within the vertebra so as to be in thermal communication with thespinal cord12. In this context, thermal communication refers to the ability of theevaporator106 to absorb heat from thespinal cord12. This thermal communication may arise from conduction, convection, or radiation. Theevaporator106 is slid along the spinal cord so as to achieve a high mutual surface area of contact.

Referring to FIG. 7, thecondenser102 is implanted just beneath theskin30 with the uninsulated side (chamber108) facing outward just underneath theskin30. One way in which to start the cooling process is to place acold pack132 over theskin30 adjacent thecondenser102. Thecold pack132 may be a thermoelectric cooler or an ice bag. Because the upper half (chamber108) is uninsulated, it is cooled by thecold pack132. The coldness condenses the coolant, which subsequently wicks through theporous separator110 and enters the lower insulated half of the disk. Because the lower half (chamber118) is insulated, the heat from the body does not allow the coolant to boil. It is noted that only a portion of the insulation of the lower chamber is shown in FIG. 7, for clarity. The coolant then flows down the capillary withinconduit104 to theevaporator106 where it boils and cools the nerve tissue. The gaseous coolant returns to theupper chamber108 of the condenser where it is cooled and liquefied, restarting the process. Removing thecold pack132 terminates the cooling.

In an alternative embodiment, shown in FIG. 8, thecondenser102 is replaced with acooling unit102′ that is resident outside the body. In this embodiment, coolingunit102′ provides and cycles a working fluid down a conduit toevaporator106.Evaporator106 may be similar in most or all aspects to the evaporator in previous embodiments. The coolant or working fluid flows back tocooling unit102′ via a return tube. The conduit and return tube may be similar to the conduit and return tube described above.

In any of the embodiments, the coolant or working fluid may be a freon or other such type of refrigerant. In the alternative embodiment of FIG. 8, the working fluid may also be saline or other similar coolants. Saline may be employed in this embodiment at least in part because this embodiment need not rely on evaporation and condensation to propel the working fluid: rather, the cooling unit may supply the required pressure.

FIG. 9 shows one possible placement of theevaporator106 along thespinal cord12. In FIG. 9, theevaporator106 is disposed along thespinal cord12 subdurally, i.e., under the dura mater. It should be noted that theevaporator106 may additionally be disposed epidurally, i.e., outside but adjacent to the dura mater.

While the invention has been described with respect to certain embodiments, it will be clear to those skilled in the art that variations of these embodiments may be employed which still fall within the scope of the invention. Accordingly, the scope of the invention is limited only by the claims appended hereto.

Claims (12)

What is claimed is:

1. A method of cooling a portion of a spinal cord of a patient, comprising:

delivering a portion of a heat pipe to a spinal cord of a patient, the heat pipe including an evaporator and a condenser, including disposing the evaporator at least in partial thermal communication with the spinal cord; and

cooling the evaporator by passing a working fluid between the evaporator and the condenser.

2. The method ofclaim 1, wherein the delivering further comprises disposing the evaporator at least in partial thermal communication with the dorsal root entry zone of the spinal cord.

3. The method ofclaim 1, further comprising passing the working fluid between the evaporator and the condenser through a conduit.

4. The method ofclaim 3, wherein the conduit is a tube.

5. The method ofclaim 3, wherein the conduit is a wick structure.

6. The method ofclaim 3, further comprising implanting the condenser within the patient.

7. The method ofclaim 3, further comprising locating the condenser externally of the patient.

8. The method ofclaim 6, wherein the condenser has an insulated lower chamber into which the conduit enters and an upper chamber into which the return tube enters, the lower and upper chambers separated by a porous structure, farther comprising passing the working fluid in gaseous form from the evaporator through the return tube within the conduit to the upper chamber, condensing the working fluid at least partially from the gaseous form into the liquid form, passing the working fluid from the upper chamber to the lower chamber through the porous structure, and passing the condensed working fluid from the lower chamber to the evaporator through the conduit.

9. The method ofclaim 8, further comprising disposing the upper chamber in thermal communication with a cold source.

10. The method ofclaim 1, further comprising disposing the evaporator adjacent the dura mater.

11. The method ofclaim 10, further comprising disposing the evaporator between the spinal cord and the dura mater.

12. The method ofclaim 10, further comprising disposing the evaporator on the side of the dura mater opposite the spinal cord.

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